Conventionally, as a control system of this kind, there has been known, for example, one disclosed in Patent Literature 1. In this control system, a so-called negative overlap amount between an exhaust valve and an intake valve in which the intake valve is opened after the exhaust valve is completely closed is set, and the valve-closing timing of the exhaust valve and the valve-opening timing of the intake valve are changed according to the set negative overlap amount to thereby control the amount of combustion gases remaining in the cylinder (hereinafter referred to as “internal EGR amount”), whereby the temperature of working medium (mixture gas of air and fuel before combustion) is controlled to a temperate at which compression ignition is possible. More specifically, according to intake air temperature, the temperature of working medium at the start of the compression stroke is calculated, and a target internal EGR amount is calculated such that the temperature of the working medium becomes a temperature at which compression ignition is possible. Then, based on the target internal EGR amount, the negative overlap amount is calculated, and according to the calculated negative overlap amount, the valve-closing timing of the exhaust valve and the valve-opening timing of the intake valve are changed.
As described above, in the conventional control system, during compression ignition, the control of the internal EGR amount is performed by negative overlap, and the valve-closing timing of the exhaust valve and the valve-opening timing of the intake valve are set according to the target internal EGR amount. Therefore, the length of time elapsed after the valve-closing timing of the exhaust valve until the top dead center position of the piston at the start of the intake stroke and the length of time elapsed after the top dead center position until the valve-opening timing of the intake valve are sometimes different from each other. In such a case, the intake valve is opened in a state in which the energy generated by the reciprocating motion of the piston is not fully cancelled out, to increase pumping loss, which causes lowering of the engine output and degradation of fuel economy. Especially, if the valve-opening timing of the intake valve is too early, the combustion gases are blown back toward the intake side, which makes the amount of intake air unstable, so that the state of combustion becomes unstable and torque variation increases, resulting in degraded drivability.
The present invention has been made to provide a solution to the above-described problem, and an object thereof is to provide a control system for an internal combustion engine, which is capable of securing excellent drivability by properly controlling the valve-opening timing of an intake valve to make stable the amount of intake air through reduction of pumping loss and the blow-back of combustion gases.
[Patent Literature 1]
Japanese Laid-Open Patent Publication (Kokai) No. 2001-289092